Séminaire du LHyGeS le jeudi 12 mars à 11h
Intervenant : Markus EGLI, Professeur à l'ETH de Zürich (Geochronology - Physical Geography Division - Department of Geography).
Titre : "Soil formation rates on silicate parent material in alpine environments: Different approaches?different results??
Lieu : EOST, 1 rue Blessig, salle de réunion Bibliothèque (2e étage)
Résumé :
Several terms regarding soil evolution have been used in literature in a partially misleading way. These include, among others, ?soil formation', ?pedogenesis', ?soil production', ?weathering', ?soil development', ?denudation' etc. As a consequence, different approaches have been developed to describe, quantify and model soil and its development.
Three important and often-used concepts are presented. The presented techniques include (1) a quantification of soil formation from chronosequences, (2) the determination of soil residence times and production rates through chemical weathering using, e.g., 230Th / 234U activity ratios and (3) a steady state approach using cosmogenic isotopes (10Be). For each method, data from different climate zones, and particularly from high-mountains (alpine environment), are compared. Independent of the chosen approach, the results seem moderately comparable, although these approaches do not determine exactly the same processes. Soil formation or production rates in high-mountain areas (alpine climate) range from very low to extremely high values and show a clear decreasing tendency with time. Very young soils have up to 3 ? 4 orders of magnitude higher rates of development than old soils (105 to 106 years). This apparently is a result of kinetic limits on weathering in regions having young surfaces and supply limits to weathering on old surfaces. In addition, alpine soils develop at least as fast as tropical soils. Thus, the concept of ?temperature-controlled' soil development in alpine regions must be reconsidered. Landscape surfaces and soils, furthermore, are known to evolve in complex, non-linear ways. As a result of changing environmental conditions over millennia, soil erosion and denudation processes also may change substantially. Results obtained from mire or lake cores indicate that major disturbances in Alpine regions have occurred about 5 ? 6 ka ago and particularly since about 2 ka.
Natural rates of soil production can serve as a basis for defining tolerable soil erosion rates. The existing guidelines for tolerable soil erosion rates are, however, often far above the natural soil production rates.